a) Calculate Pr(Y1 > 20) using the given distribution.

b) Get the pdf for Y = Y1 + Y2 where Y1 and Y2 are i.i.d. random variables and each has an exponential distribution with the parameter Θ=9. Now calculate Pr(Y > 20).

c) Get the pdf for Y = Y1 + Y2 + Y3 where Y1, Y2 and Y3 are i.i.d. random variables and each has an exponential distribution with the parameter Θ=9. Now calculate Pr(Y > 20).